We determined the structure of the a T cell receptor (TCR) b chain complexed with the superantigen staphylococcal enterotoxin B (SEB) to 2.4 resolution. We also determined the structure, to 2.6 resolution, of the complex between the TCR b chain and a mutant of SEB in which valine at position 26 is replaced by tyrosine (SEB V26Y). The crystals belong to space group P21 with cell dimensions a = 71.2 , b = 83.6 , c = 83.0 for the wild type b-SEB complex and a = 70.9 , b = 83.0 , c = 82.8 for the mutant b-SEB V26Y complex. There are two complex molecules in the asymmetric unit. X-ray diffraction data up to 2.4 (b-SEB) and 2.6 (b-SEB V26Y) were collected at 100 oK from one flash-cooled crystal for each complex using synchrotron radiation at CHESS beamline F-1 with a Princeton 2K CCD detector. The crystals were soaked in 10% PEG 8000, 24% glycerol, and 0.1 M Tris-HCl, pH 8.5, prior to flash-cooling in liquid nitrogen. Data were integrated and merged using HKL/DENZO/SCALEPACK which gives 34,943 unique reflections with Rmerge= 9.2% for b-SEB and 28,757 unique reflections with Rmerge= 8.1% for b-SEB V26Y. The data sets are 91.5% complete to 2.4 for b-SEB (79.1% from 2.5-2.4 ) and 97.0% complete to 2.6 for b-SEB V26Y (90.6% from 2.7-2.6 ). The structure of the wild type b-SEB complex was solved by the molecular replacement method with the program AMoRe (Navaza, 1994). The search models consisted of the 14.3.d TCR b chain refined at 1.7 resolution (PDB accession code 1bec) and SEB refined at 1.9 resolution (PDB accession code 1SE4). The structure was refined by iterative cycles of simulated annealing and temperature factor (B) refinement using X-PLOR interspersed with model building into Fo- Fc and 2Fo- Fc electron density maps using TURBO-FROD. The final model contains 7,589 protein atoms and 179 water molecule with Rfree= 0.309 and Rwork= 0.228 in the range 6-2.4 . The r.m.s. deviations from ideal bond lengths and bond angles are 0.006 and 1.79o, respectively. The structure determination of the b-SEB V26Y mutant complex was begun from the partially refined structure of the b-SEB wild type complex. The final model of the b-SEB V26Y complex contains 6,915 protein atoms and 31 water molecule with Rfree= 0.326 and Rwork= 0.229 in the range 6-2.6 . The r.m.s. deviations from ideal bond lengths and bond angles are 0.008 and 1.37o, respectively.